This invention relates to a deflection sensor and relates particularly, though not exclusively, to a deflection sensor for a taut wire perimeter fence detection system and the system per se.
A taut wire fence operates as a number of parallel wires attached between two anchor posts. Each of the wires acts as a spring if there is an attempt to move a wire. Attached to each wire is a sensor which when displaced, causes an alarm. Also, cutting any wire causes the sensor to be displaced by tension from the uncut section of the wire. Such a system has a very low false alarm rate and is not disturbed in most environmental conditions. The use of a barbed wire also poses a deterrent to intrusion. A fence structure is also a barrier which clearly defines a perimeter or border. Such systems have been available for many years and examples of these systems may be found in U.S. Pat. Nos. 5,392,027, 5,602,534, 5,268,672, 4,730,809, 4,500,873 and 4,367,459.
In a taut wire fence installation, the sensor post is installed and, depending on the system, the wire sensors are mechanically disconnected from the fence fabric allowing the correct tension to be applied to each fence wire. Once the fence fabric is correctly tensioned and stable, the sensors are mechanically coupled to the fabric of the fence. It is during this stage that the wire sensors are most vulnerable. Incorrect tension settings may cause the sensor to be over strained and in the process damaged or destroyed. Systems employing a sensor that is transverse to the strained fence or wire are particularly sensitive to this action.
It is an object of the present invention to provide a sensor that can be installed after the fence wire has been installed.
A further object of the invention is to provide a sensor that is easy to install.
A still further object is to provide a sensor that operates in line with the wire tension.
With these and other objects in view the present invention provides a deflection sensor for a taut wire type detection system, said deflection sensor including a plate member adapted to be pivotally mounted, a first wire attachment point at one end of said plate member, a second wire attachment point remote from said first wire attachment point and a transducer or sensor element located on said plate member between said attachment points.
Preferably said attachment points are at opposite ends of said plate member and/or on opposite sides of said plate member. In a practical embodiment said attachment points are formed from tabs bent from said plate member to extend on opposite sides of said plate member and said tabs include a slot for reception of said wire. Preferably said plate member includes a further slot at the bottom of said plate member to guide the wire from one side to the other side of said plate member. Preferably a cut out or slot is formed in said plate member adjacent said transducer to increase the sensitivity of said sensor. In a further practical embodiment said sensor is pivotally mounted on a shock absorber means. The transducer or sensor element is preferably a strain gage device.
The invention also relates to a taut wire type detection system including at least one taut wire for a perimeter fence supported by a plurality of posts, at least one deflection sensor of the type previously defined being pivotally mounted to one of said posts or a support thereon and a sensor processing circuit for interrogating said at least one deflection sensor and to provide an alarm indication on tampering of said at least one taut wire or said at least one deflection sensor.